The present invention relates to an objective lens device which can be applied to a plurality of optical disks having different thicknesses, and to an optical pickup adopting the same.
Generally, an optical pickup of a compact disk player (CDP), a digital video disk player (DVDP), etc., is used to record and reproduce information on and from a recording medium in a non-contact manner.
Preferably, the optical pickup adopted in the DVDP which enables high-density recording and reproduction is applied to a recording medium such as a digital video disk (DVD), a compact disk (CD) or a CD-ROM for compatibility.
However, the standard thickness of the DVD is different from that of the CD or CD-ROM due to an allowance in the inclination of the disk and the numerical aperture of an objective lens. That is, since the thickness of the DVD is different from that of the CD, a spherical aberration occurs when an optical pickup for a DVD is applied to the CD. In this case, sufficient light intensity for recording an information signal cannot be obtained or a reproduced signal is deteriorated by the spherical aberration.
In order to solve the above problems, an objective lens device and an optical pickup adopting the objective lens device are proposed as follows.
In an objective lens device proposed by the applicants of the present invention, light of a middle region between a near axis region in which the spherical aberration occurring around the central optical axis does not occur and a far axis region surrounding the near axis region is blocked, an optical spot having a small peripheral light is formed without interference of light in the middle region.
As shown in FIGS. 1 and 2, a light control device is provided for blocking or scattering a light 122 of an incident light 120, incident on the middle region between the near axis region and the far axis region. Referring to FIG. 1, the light control device includes a transparent member 110 having a light control layer 111 for blocking or scattering the light incident on the middle region. In FIG. 2, the light control device includes a light control hole 111' formed in an objective lens 100', for scattering the light 122 incident on the middle region.
The near axis region is a region around the central axis of the lens (optical path), where negligible spherical aberration exists without influence on the central rays of the incident light 121. The far axis region is a region which is farther from the optical path than the near axis region and in which a light 123 is incident, thereby affecting the central rays. Also, the middle region is a region between the near axis region and the far axis region.
FIG. 3 is a schematic diagram showing an example of an optical pickup adopting the objective lens device shown in FIG. 1.
The optical pickup adopts the transparent member 110 having the light control layer 111 which was described with reference to FIG. 1. Thus, light emitted from a light source 150 passes through a beam splitter 140 and is then collimated by a collimating lens 130 to be incident on an objective lens 100 parallel to an optical path. At this stage, the light 122 incident on the middle region is blocked by the light control layer 111 formed on the transparent member 110, so that only the lights incident on the near axis region and the far axis region, respectively, pass through the objective lens 100. The lights 121 and 123 are converged by the objective lens 100 to form an optical spot on an optical disk 10. Thus, the objective lens device is compatible for a plurality of disks 10 having different thicknesses, such as a CD and a DVD.
Then, the light is reflected from the optical disk 10 and received by a photodetector 170 via the objective lens 100, the collimating lens 130 and the beam splitter 140. The photodetector 170 is composed of at least four divided plates which convert the received light into an electrical signal to be used as an error signal or information signal. At this stage, a condensing lens 160 is disposed between the beam splitter 140 and the photodetector 170.
FIG. 4 is a schematic diagram showing an example of the optical pickup adopting the objective lens device shown in FIG. 2.
The optical pickup adopts an objective lens 100' having a light control hole 111' for scattering the light 122 incident on the middle region. In this case, the elements designated by the same reference numerals as those of FIG. 3 and which are not described represent the same elements as those illustrated with reference to FIG. 3.
FIG. 5 is a graph showing an initial focus error of the optical pickup adopting a CD as a recording medium. The optical pickup includes an objective lens having a numerical aperture of 0.6 and a diameter of 4.04 mm, and the objective lens has an annular light control hole having an inner diameter of 1.2 mm and a width of 0.15 mm.
In FIG. 5, the X-axis represents the degree of defocus and the Y-axis represents a detected voltage value as an index of the degree of error.
Also in FIG. 5, there are two portions A and B, where the detected voltage values are zero. The portion A occurs when the optical pickup is placed in an "on focus" position to control an initial focus position. On the contrary, the portion B is occurs due to the spherical aberration caused by the difference in thickness of the recording media, and a waveform near the portion B, including the portion B, is defined as a parasitic waveform. The parasitic waveform exists because the focusing point of the beam which passed the far axis region is formed over a broad region by the spherical aberration of the objective lens.
When the parasitic waveform exists, the portion B may be considered as the "on focus" position due to the mechanical vibration of the recording medium.